Insulation-coated electric conductor

ABSTRACT

A multilayer insulation-coated electric conductor for a coil constituting a circuit by forming a welded portion in the coil, wherein at least one layer of the multilayer insulation-coating layers of the coated metal conductor is composed of at least one resin selected from the group consisting of a polyimide resin, a polyesterimide resin, and an H-class polyester resin.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/525,775 filed Sep. 22, 2006 which is a continuation of U.S. patentapplication Ser. No. 11/138,740 filed May 25, 2005 which is acontinuation of U.S. patent application Ser. No. 09/969,291 filed onOct. 1, 2001 each entitled Insulation-Coated Electric Conductor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an insulation-coated electricconductor.

2. Description of the Related Art

A conductor coated with an electrical insulating material isincorporated in a variety of electric machinery and tools and used inlarge quantity for uses for coils. It is especially abundantly used inelectric machinery and tools as represented by motors and generators. Awinding, having a conductor whose cross-section has a non-round shape,and being spirally covered with, as a coating material, an insulatingmaterial, such as glass and paper, has conventionally been used. Such awinding has been used for parts of machinery and tools required to haveextremely high reliability, such as for coils for generators of powerplants, coils for transformers, coils for vehicular driving motors, andthe like.

Recently, a winding having a conductor whose cross-section has anon-round shape, such as an approximately rectangular shape, tends to beused also in machinery and tools smaller than the machinery and toolsdescribed above. Even in these small machinery and tools, they arerequired to be made high performance in a shape of coil. Accordingly, amethod for forming a coil by joining coated conductors together eachhaving a cross-section shape corresponding to the coil shape, tends tobe employed as the coil production method, in stead of the conventionalmethod for producing a coil by winding an electric wirecircumferentially with a coil, a so-called wire-winding. Employment ofsuch a winding of a conductor with a shape other than round in a smallmachinery and tools is developed, because a gap between the winding anda core of the coil can be eliminated and magnetic field loss can belessened, to resultantly improve performance of the coil, and alsobecause the coil to be used in a small machinery and tools can furtherbe miniaturized. Making the conductor have a shape other than roundmakes it difficult to wind a long electric wire, as it is, directly ontoa core of a coil, when forming a coil-like circuit. Therefore, atechnique which has recently been employed involves steps of, at first,forming short conductors into partial shapes of a coil, and then formingan entire circuit by welding the conductors to join together.

To form such a coil, the conductors are required to be connected to oneanother. To connect the conductors, the parts where soldering hasconventionally been conducted are to be subjected to an electric weldingmethod, such as fusing (electric welding while applying pressure), TIGwelding, and the like. This is because, it is recently required to use amaterial having the same level of performance as a conductor (such ascopper) currently used, as a connection material, since with aconventionally employed soldering, a serious environmental effect iscaused by lead or the like contained in the solder, when used productsare discarded, and also the soldered parts are inferior in reliabilityto vibration of machinery and tools.

Conventionally, as coating materials to be used for round enameledwires, a variety of resins, such as polyesters, have been used. However,since the conductors are to be connected to one another by directlyheating the conductors and melting them in fusing and TIG welding, theinsulation coating on the periphery of the connected portion is heatedto an extremely high temperature, to receive considerable thermaldeterioration. For example, to connect copper pieces to each other byusual welding, the temperature of the copper pieces must be increased tothe melting point of copper or higher, and for that, the conductortemperature is increased to about 1,100° C. or higher. The increase ofthe conductor temperature causes thermal deterioration of the insulationcoating on the periphery of the connected portion of the conductors, andfurther, low-molecular-weight components in the coating material areevaporated by the heat, to result in blistering (foaming) in thecoating. As a result, the electric properties of the coating material onthe periphery of the connected portion are possibly deteriorated in somecases. That such welding heat affects the coating has been well known,and to lessen the effect is necessary also, to improve the reliabilityof electric machinery and tools. If the conductor temperature reaches1,100° C. in a welded portion, the heat that a coating, required to haveinsulating properties, receives reaches as high as about 600° C. at 10mm distance from the welded portion. Further, the above-describedblistering of the coating cannot be avoided if polyamideimide resin,which has conventionally been used, is used solely.

SUMMARY OF THE INVENTION

The present invention is a multilayer insulation-coated electricconductor, which can be used in a coil that constitutes a circuit byforming a welded portion in the coil, wherein at least one layer of themultilayer insulation-coating layers of the coated metal conductor iscomposed of at least one resin selected from the group consisting of apolyimide resin, a polyesterimide resin, and an H-class polyester resin.

Other and further features and advantages of the invention will appearmore fully from the following description.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is an illustration of smaller conductors being welded to form alarger coil.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The inventor of the present invention has paid attention to thematerials of coatings of coated electric conductors, and found a novelcoating constitution that is durable to heat at the time of welding, andthat causes no abnormality, such as blistering in the coating.

Regarding the coating constitution durable to the heat at the time ofwelding, the present inventor focused on the only momentary applicationof heat at the time of welding, and investigated heat resistance(instantaneous heat resistance) of the coating. The present inventor hasconfirmed the following with respect to the instantaneous heatresistance of the coating: That heat is conducted from the conductorside at the time of welding the conductor; that the material of thecoating formed nearest to the conductor side is most deteriorated by theheat; and that the decomposition gas generated in the coating materialin the conductor side causes voids and blisters (fine foams) in theentire coating layers. Therefore, to deal with the occurrence of voidsor the like, the present inventor has studied to use a material that isnot softened even if coating is heated, for some parts of the coating,and I have consequently found that the above-described conventionalproblems can be solved by using a specific material (a polyimide,polyesterimide, or H-class polyester resin) partially for the coating.The present invention has been achieved based on these findings.

That is, according to the present invention, there is provided thefollowing means:

1) A multilayer insulation-coated electric conductor for a coilconstituting a circuit by forming a welded portion in the coil as shownin FIG. 1, wherein at least one layer of the multilayerinsulation-coating layers of the coated metal conductor is composed ofat least one resin selected from the group consisting of a polyimideresin, a polyesterimide resin, and an H-class polyester resin;

2) The coated metal conductor according to the item (1), wherein theuppermost layer of the multilayer insulation-coating layers is composedof a polyamideimide resin, and wherein an under insulation-coating layeradjacent to the uppermost layer is composed of at least one resinselected from the group consisting of a polyimide resin, apolyesterimide resin, and an H-class polyester resin;

3) The coated metal conductor according to the item (1) or (2), whereinthe lowermost layer of the insulation-coating layers of the multilayerinsulation-coated electric conductor is composed of a polyamideimideresin;

4) The coated metal conductor according to any one of the items (1) to(3), wherein the metal conductor is low-oxygen copper with an oxygencontent of 30 ppm or lower, or an oxygen-free copper; and

5) The coated metal conductor according to any one of the items (1) to(4), wherein a cross-section of the conductor has a shape other thanround.

The reasons why the functions/effects of the present invention can beexhibited are not made clear, but it is assumed that the polyimide resinis less in decrease of elastic modulus even at a high temperature ascompared with that at a room temperature, and that the polyesterimideresin and the H-class polyester resin have high initial elastic modulusand are scarcely softened even at a high temperature.

The polyimide resin that can be used for forming a part of the coatinglayers in the present invention, is not particularly restricted, and usecan be made of the well-known polyimide resins, such as aromaticpolyimides (API) and thermosetting aromatic polyimide. For example, usecan be made of either commercially available products (e.g. trade name,#3000, produced by Toray-Du Pont Co., Ltd.) or those that can beobtained by imidation of a polyamide acid by heating at the time ofbaking in the coating formation, by using a polyamide acid solutionobtainable by reacting an aromatic tetracarboxylic acid dianhydride andan aromatic diamine in a polar solvent, in a usual manner.

Further, as the polyesterimide resin, use can be made of the well-knownpolyesterimide resins that can be obtained by introducing an ester bondinto the main chain of a polyimide. As a commercially available product,for example, Isomid 40SH (trade name, produced by Nisshoku SchenectadyKagaku Inc.) can be mentioned.

In the present invention, the H-class polyester resin means an aromaticpolyester that is modified by adding, for example, a phenol resin andthat has heat resistance of H-class (IEC). As the H-class polyesterresin commercially available, mention can be made of Isonel200 (tradename, produced by U.S. Schenectady International Co.)

Further, as the polyamideimide resin that can be used for forming theuppermost layer or the lowermost layer, use can be made of either acommercially available product (for example, trade name, H1406, producedby Hitachi Chemical Co., Ltd.), or those that can be, for example,obtained by direct reaction of a tricarboxylic acid anhydride and adiisocyanate in a polar solvent or obtained by reacting tricarboxylicacid anhydride with a diamine in a polar solvent to introduce imidebonds and then carrying out amidation with a diisocyanate, in a usualmanner. The polyamideimide resin is low in thermal conductivity and highin dielectric breakdown voltage, as compared with other resins, and itis possible to be hardened by baking.

In the coated metal conductor of the present invention, as describedabove, the foregoing polyamideimide resin can be used in the uppermostlayer among the insulation-coating layers. By using the polyamideimideresin in the uppermost layer adjacent to an insulation-coating layercomposed of at least one resin selected from the group consisting of thepolyimide resin, the polyesterimide resin, and the H-class polyesterresin, even if being subjected to strong bending, the resultant coatinglayers can solve the trouble caused by film elongation, such as crazingand cracks of film.

As the resin for forming the uppermost layer of the coating of theconductor of the present invention, use can be also made of aself-lubricating resin containing wax or a lubricant mixed with ordispersed in, by a usual manner. As the wax that can be used for that,any commonly employed one can be employed without any restrictions, andthe examples thereof include synthetic waxes, such as polyethylene wax,petroleum wax, and paraffin wax, and natural waxes, such as carnaubawax, candelilla wax, and rice wax. Also as the lubricant, any one ofknown lubricants can be employed without particular restrictions, andthe examples of the lubricant that can be employed include silicone,silicone macromonomers, and fluororesins. In the coated electricconductor of the present invention, the formation method of the resinfor forming each resin layer of the coating is not particularlyrestricted and a variety of known methods can be employed.

Further, in the insulation-coated electric conductor of the presentinvention, preferably, the lowermost layer among the insulation-coatinglayers is composed of the polyamideimide resin, and through anotherresin layer or directly on the lowermost layer, an insulation-coatinglayer composed of at least one resin selected from the group consistingof the polyimide resin, the polyesterimide resin, and the H-classpolyester resin, can be provided. According to the resultant coatedmetal conductor having the constitution of the above insulation-coatinglayers, such function is exhibited that the insulation film is difficultto cause thermal deterioration, to the heat transmitted from theconductor side when the insulation-coated electric conductor is employedin a motor or a transformer. The above another resin layer (intermediatelayer) may be composed of any resin without particular restriction, butit is preferable to use, for example, the above-described at least oneresin selected from the group consisting of the polyimide resin, thepolyesterimide resin, and the H-class polyester resin, as the anotherresin.

In the insulation-coated electric conductor of the present invention,the number of the layer of the insulation-coating layers formed on theelectric conductor is not particularly limited, but the layerconstitution is preferably composed of four layers or less, morepreferably three or two layers.

A resin varnish for coating is coated and baked on the conductor, andthe coated electric conductor can be obtained.

As a conductor, a conventionally known conductor can be used. Preferablylow-oxygen copper with an oxygen content of 30 ppm or lower, morepreferably low-oxygen copper with an oxygen content of 20 ppm or lower,or oxygen-free copper can be used as a conductor. If the oxygen contentis 30 ppm or lower, when the conductor is melded by heat for welding, novoid due to the oxygen contained in the copper is occurred in the weldedportion. Therefore the electric resistance in the welded portion can beprevented from being deteriorated. In addition, the mechanical strengthof the welded portion can be maintained.

Further, conductors having any desired cross-sectional shape can beused. The conductor having a shape other than round is preferably used,and especially preferably the conductor of a rectangular shape is used.

A usual manner can be employed as the method for coating a resin varnishcontaining the above resin(s) onto the conductor. For example, a methodusing a die having a similar shape to the conductor shape can beemployed for varnish coating, or if the cross-section shape of theconductor is square, a die, a so-called “universal die”, formed in alattice form may be employed. The conductor coated with the resinvarnish can be baked in a baking furnace also by a usual manner.Although depending on the shape and the like of the furnace to beemployed, concrete baking conditions are set to be at a temperature of400 to 500° C. with a passing time of 30 to 90 seconds to completebaking, if a natural convection-type vertical furnace of about 5 m infurnace length.

In the present invention, the thickness of the layer composed of atleast one of the foregoing polyimide resin, polyesterimide resin, andH-class polyester resin is not particularly restricted, but it ispreferably 4 to 35 μm, more preferably 5 to 18 μm. Further, the totalthickness of the polyamideimide resin layer is preferably 10 to 40 μmand more preferably 10 to 35 μm, in a whole coating.

The thickness of the entire coating film is generally about 15 to about55 μm and preferably 25 to 50 μm.

The insulation-coated electric conductor of the present invention ispreferable for composing a coil in a motor or a generator, and it isdurable to welding heat when composing a circuit by providing a weldedportion in the circuit of the coil.

Further, the coated electric conductor of the present invention hasexcellent instantaneous heat resistance, and it causes no void orblister in the coating film even at the time of working such as weldingof the conductor in which heat at a high temperature is applied undersevere conditions during the coil production process, and goodproperties (soundness) can be maintained, so that the resultantinsulated wire in which the insulation-coated electric conductor is usedcan be protected from deterioration. Further, in the case of using thecoated metal conductor, thermal deterioration of the insulation-coatingfilm hardly takes place even by the heat transmitted from the conductorside, thereby a coil high in reliability can be provided. The abovematters consequently result in such excellent effects to be exhibitedthat the coated metal conductor contributes to provide the entiremachinery and tools themselves using the coil, in which the coated metalconductor of the present invention is used, with improved performanceand with increased reliability to the machinery and tools.

The present invention will be described in more detail based on thefollowing examples, but the invention is not limited to these.

EXAMPLES

The constitutions of the resin coatings of the following examples andcomparative examples are collectively shown in Table 1 and Table 2. Theresults of the evaluation tests for the resin coated conductors obtainedin the following manners are shown in Table 3 to Table 5.

Example 1

A polyamideimide resin (PAI) (trade name H1406, produced by HitachiChemical Co., Ltd.), a polyimide resin (PI) (trade name #3000, producedby Toray-Du Pont Co., Ltd.), and a polyamideimide resin (PAI) (tradename H1406, produced by Hitachi Chemical Co., Ltd.) were used forforming coatings, successively from the lowermost layer in this order,on a rectangular conductor (copper with oxygen content of 15 ppm)chamfered in four corners with the radius r=0.5 mm and having the size1.8 mm×2.5 mm (thickness×width), and the total thickness of the coatingfilm was made to be 45 μm. The thickness of each coating was as shown inTable 1. A plurality of dies with similar shape to the conductor shapewere used for the coating-film formation, and in a baking furnace of 8-mfurnace length, baking at 450° C. with a baking time for about 15seconds was repeated a plurality of times, to obtain a resin-coatedconductor. The results of the following evaluation tests carried out forthe resin-coated conductor are shown in Table 3.

Examples 2, 3

The resins were used in the same manner as those of the above Example 1,and the baking conditions for the resin-coating were also made the sameto the Example 1. However, the thickness of each coating was changed asshown in Table 1. The results of the evaluation tests carried out forthe thus-obtained resin-coated conductors are shown in Table 3.

Comparative Example 1

A polyamideimide resin (PAI) (trade name H1406, produced by HitachiChemical Co., Ltd.) was used for forming a coating, on a rectangularconductor (copper with the same quality as that of Example 1) chamferedin four corners with the radius r=0.5 mm and having the size 1.8 mm×2.5mm, and the total thickness of the coating was made to be 45 μm. Aplurality of dies with similar shape to the conductor shape were usedfor the coating-film formation, and in a baking furnace of 8-m furnacelength, baking at 450° C. with a baking time for about 15 seconds wasrepeated a plurality of times. The results of the evaluation testscarried out for the thus-obtained resin-coated conductor are shown inTable 3. TABLE 1 Example 1 Example 2 Example 3 Comparative example 1Conductor size 1.8 × 2.5 (mm × mm) Radius of four 0.5 corners ofconductor (mm) Total coating 45 45 45 45 thickness (μm) Coatingthickness of PAI (HI406) PAI (HI406) PAI (HI406) PAI (HI406) lower layer(μm) 18 25 25 30 Coating thickness of PI (3000) PI (3000) PI (3000) —middle layer (μm) 17 10  5 Coating thickness of PAI (HI406) PAI (HI406)PAI (HI406) PAI (HI406) upper layer (μm) 10 10 15 15

Example 4

An H-class polyester resin (HPE) Isonel200 (trade name, produced by U.S.Schenectady International Co.) and a polyamideimide resin (PAI) (tradename H1406, produced by Hitachi Chemical Co., Ltd.) were used forforming coatings, in this order from the lowermost layer, onto arectangular conductor (copper with oxygen content of 20 ppm) chamferedin four corners with the radius r=0.8 mm and having the size 2.0 mm×3.0mm, and the total thickness of the coatings was made to be 50 μm. Thethickness of each coatings is shown in Table 2. A plurality of dies withsimilar shape to the conductor shape were used for the coating-filmformation, and in a baking furnace of 8-m furnace length, baking at 450°C. with a baking time for about 20 seconds was repeated a plurality oftimes. The results of the evaluation tests carried out for thethus-obtained resin-coated conductor are shown in Table 4.

Example 5

The resins were used in the same manner as those of the above Example 4,and the baking conditions for the resin coatings were also the same tothe Example 4. However, the thickness of each coating was changed asshown in Table 2. The results of the evaluation tests carried out forthe thus-obtained resin-coated conductor are shown in Table 4.

Comparative Example 2

An H-class polyester resin (HPE) Isonel200 (trade name, produced by U.S.Schenectady International Co.) was used for forming a coating, onto arectangular conductor (copper with the same quality as that of Example4) chamfered in four corners with the radius r=0.8 mm and having thesize 2.0 mm×3.0 mm, and the total thickness of the coating was made tobe 50 μm. A plurality of dies with similar shape to the conductor shapewere used for the coating-film formation, and in a baking furnace of 8-mfurnace length, baking at 450° C. with a baking time for about 20seconds was repeated a plurality of times. The results of the evaluationtests carried out for the thus-obtained resin-coated conductor are shownin Table 4.

Example 6

A polyamideimide resin (PAI) (trade name H1406, produced by HitachiChemical Co., Ltd.), a polyesterimide resin (PEI) Isomid 40SH (tradename, produced by Nisshoku Schenectady Kagaku Inc.), and apolyamideimide resin (PAI) (trade name H1406, produced by HitachiChemical Co., Ltd.) were used, in this order from the lowermost layer,for successively forming a three-layer coating structure, onto arectangular conductor (copper with oxygen content of 15 ppm) with theradius r=0.6 mm in four corners chamfered and having the size 1.5 mm×2.4mm, and the total thickness of the coatings was made to be 35 μm. Thethickness of each of the coatings is shown in Table 2. A plurality ofdies with similar shape to the conductor shape were used for thecoating-film formation, and in a baking furnace of 8-m furnace length,baking at 450° C. with the baking time of about 20 seconds was repeateda plurality of times. The results of the evaluation tests carried outfor the thus-obtained resin-coated conductor are shown in Table 5.

Example 7

A polyesterimide resin (PEI) Isomid 40SH (trade name, produced byNisshoku Schenectady Kagaku Inc.) and a polyamideimide resin (PAI)(trade name H1406, produced by Hitachi Chemical Co., Ltd.) were used forsuccessively forming coatings, in this order from the lowermost layer,onto rectangular conductor (copper with the same quality as that ofExample 6) with the radius r=0.6 mm in four corners chamfered and havingthe size 1.5 mm×2.4 mm, and the total thickness of the coatings was madeto be 35 μm. The thickness of each coatings is shown in Table 2. Aplurality of dies with similar shape to the conductor shape were usedfor the coating-film formation, and in a baking furnace of 8-m furnacelength, baking at 450° C. with the baking time of about 20 seconds wasrepeated a plurality of times. The results of the evaluation testscarried out for the thus-obtained resin-coated conductor are shown inTable 5.

Comparative Example 3

A polyester imide resin (PEI) Isomid 40SH (trade name, produced byNisshoku Schenectady Kagaku Inc.) was used for forming a coating, onto arectangular conductor (copper with the same quality as that of Example6) with the radius r=0.6 mm in four corners chamfered and having thesize 5 mm×2.4 mm, and the total thickness of the coating was made to be35 μm. A plurality of dies with similar shape to the conductor shapewere used for the coating-film formation, and in a baking furnace of 8-mfurnace length, baking at 450° C. with the baking time of about 20seconds was repeated a plurality of times. The results of the evaluationtests carried out for the thus-obtained resin-coated conductor are shownin Table 5. TABLE 2 Comparative Comparative Example 4 Example 5 example2 Example 6 Example 7 example 3 Conductor 2.0 × 3.0 1.5 × 2.4 size (mm ×mm) Radius of 0.8 0.6 four corners of conductor (mm) Total 50 50 50 3535 35 coating thickness (μm) Coating HPE HPE HPE PAI PEI PEI thicknessof (Isonel200) (Isonel200) (Isonel200) (HI406) (Isomid40SH) (Isomid40SH)lower layer 40 25 50 10 25 35 (μm) Coating — — — PEI — — thickness of(Isomid40SH) middle layer 15 (μm) Coating PAI PAI — PAI PAI — thicknessof (HI406) (HI406) (HI406) (HI406) upper layer 10 25 10 10 (μm)Evaluation MethodsBending (Edgewise Bending)

Bending (edgewise bending) was carried out at 180° in the direction ofthe edge face of the coated conductor. The bending radius was made to bethe same as the width in the width-wise (transverse) direction of theconductor (1-w bending). After such bending was carried out, a pin holetest according to JIS C3003 was carried out, to examine occurrence ofpin holes. The term, “good”, means no coating crack was observed whenbending and no pin hole was formed.

Instantaneous Heat Resistance (Fusing)

The dry area (spot) of the coating film immediately neighboring to thewelded portion was observed when welding was carried out in the electriccurrent conditions, as described in Tables 3 to 5, while crossingorthogonally the flat faces of the two coated conductors brought intocontact with each other and sandwiching the crossing portion withelectrodes from the upper and lower sides. The term, “good”, means novoid was observed and no burning took place.

Instantaneous Heat Resistance (TIG Welding)

The coating only at one end portion in 5-mm length of the two coatedconductors was peeled, and the resultant conductors were alignedparallel to fix the peeled faces with the edge faces thereof beingbrought into contact with each other, and the resulting butt faces ofthe conductors were welded by TIG welding. The conditions are as shownin Tables 3 to 5. The dry area (spot) of the coating film immediatelyneighboring to the welded face portion in that case was observed. Theterm, “good”, means no void was observed and no burning took place.

Dielectric Breakdown Voltage

The test was carried out employing a metal foil method as described inJIS C3003. The average values of the results n=5 are shown in Tables 3to 5. Separately, the test was carried out for samples after left tostand in a thermostatic chamber at 230° C. for 5 days. TABLE 3 Example 1Example 2 Example 3 Comparative example 1 Bending property Good GoodGood Cracks of the coating Edgewise bending film were observed. 2.5 φmm180° (Pin hole) (None) (None) (None) (Pin holes were occurred.)Instantaneous heat resistance Good Good Good Voids were observed Fusingin the coating film. 200 A × 0.5 second Instantaneous heat resistanceGood Good Good Voids about 1 mm TIG welding size were observed. 220 A ×0.5 second Dielectric breakdown voltage 6.5 6.8 7.0 6.5 (kV) Dielectricbreakdown voltage 5.8 6.2 6.3 4.4 after 230° C. × 5 days (90)   (91)  (90)   (68)   (kV) (remaining ratio: %)

TABLE 4 Exam- ple 4 Example 5 Comparative example 2 Bending propertyGood Good Cracks of the coating film Edgewise bending were observed. 3.0φmm 180° (Pin hole) (None) (None) (Pin holes were occurred.)Instantaneous heat Good Good Voids were observed in the resistanceFusing coating film. 200 A × 0.5 second Instantaneous heat Good GoodVoids about 2.0 mm size resistance TIG welding were observed. 220 A ×0.5 second Dielectric breakdown 7.0 7.3 7.2 voltage (kV) Dielectricbreakdown 5.3 5.8 2.4 voltage after 230° C. × (75)   (79)   (33)   5days (kV) (remaining ratio: %)

TABLE 5 Exam- Exam- Comparative ple 6 ple 7 example 3 Bending propertyGood Good Cracks of the Edgewise bending coating film were 2.4 φmmobserved. 180° (Pin hole) (None) (None) (Pin holes were occurred.)Instantaneous heat resistance Good Good Voids were Fusing observed inthe coating 200 A × 0.5 second film. Instantaneous heat resistance GoodGood Voids about 2.5 mm TIG welding size were observed. 220 A × 0.5second Dielectric breakdown voltage 5.7 4.8 5.1 (kV) Dielectricbreakdown voltage 4.3 4.1 2.6 after 230° C. × 5 days (90)   (77)  (51)   (kV) (remaining ratio: %)

Having described our invention as related to the present embodiments, itis our intention that the invention not be limited by any of the detailsof the description, unless otherwise specified, but rather be construedbroadly within its spirit and scope as set out in the accompanyingclaims.

1. A method of making an insulated electrical conductor comprising:coating a conductor having an oxygen content of 30 ppm or lower with atleast one resin selected from the group consisting of a polyimide resin,a polyesterimide resin, and an H-class polyester resin, so as to form aresin underlayer; and coating the underlayer with a polyamideimide resinso as to form an outermost resin layer.
 2. The method of claim 1,further comprising coating the conductor with a polyamideimide resin toform a lowermost resin layer prior to forming the resin underlayer. 3.The method of claim 1, wherein the conductor cross-section is a shapeother than round.
 4. The method of claim 1, wherein the conductorcross-section is a rectangular shape.
 5. The method of claim 1, whereinwax or a lubricant is dispersed in or mixed with a resin forming theoutermost resin layer.